1. Field of the Invention
The present embodiments relate to a package substrate and process for making the same, and more particularly to a package substrate with pillars and a process for making the same.
2. Description of the Related Art
Package substrates provided with a plurality of pillars for connecting the solder bumps of a semiconductor die have been proposed. After a reflow process, solder joints are formed between the die and the pillars, so that the die is bonded to the pillars and the electrical connection therebetween is secured. The pillars are typically formed by electroplating. However, non-predictable and variable plating parameters in the plating bath often leads to over-plating or under-plating, which, in turn, results in the top surfaces of the plated pillars not being coplanar. The coplanarity issue negatively affects the solder joint reliability after packaging. Fine-pitch solder bumps, wafer level packaging (WLP), and large-scale substrates are particularly sensitive to this issue. The coplanarity issue mainly results from the non-uniform electric current density distribution, which is especially serious on micro-scale patterns. This non-uniform electric current density distribution is not caused by only one factor, but by various plating parameters such as plating bath design, chemical additives, magnitude of current density, use of current type, distance between cathode and anode, agitation method, chemical maintenance, pre-cleaning solution, configurations, arrangements and volumes of patterns, high aspect ratio, and so on. Current manufacturing processes make it difficult to control the height deviation of the pillars to within a range of 5 μm.